Creating an error in an else - php

Question

Basically I want to add one last piece of validation, if nothing is selected on the items page then an error appears or the user is returned to another page.
When submit is selected the form action sends it to the confirm page and the below is executed which displays the items selected if there is 1 or more entered if ($partno == $varname & $qty > 0) but I dont no what to put in the else part to return an error or take the user back to the previous page.
<?php
$visitor = $_POST['visitor'];
echo "<p>" . 'Hello ' . "<b>" . $visitor . "</b> " . 'please confirm your purchase(s) below.' . "</p>";
if (!($data = file('items.txt'))) {
echo 'ERROR: Failed to open file! </body></html>';
exit;
}
$total = 0;
foreach ($_POST as $varname => $varvalue) {
$qty = $varvalue;
foreach ($data as $thedata) {
list($partno, $name, $description, $price, $image) = explode('|', $thedata);
if ($partno == $varname & $qty > 0) {
echo "<tr><td><img src='$image' width='50' height='50' alt='image'</td>
<td>$partno<input type='hidden' name='$partno' value=$partno></td><td>$name</td><td>£$price</td>
<td> $qty</td><td><input type='hidden' name='visitor' value=$visitor></td>
<td><input type='hidden' name='qty' value=$qty></td></tr>";
$total = $total + $price * $qty;
} else {
}
}
}
?>

Answer 1

You'd have something like this:
$errors = array();
foreach(...) {
if ($partno == $varname & $qty > 0) {
... code for "ok" stuff
} else {
$errors[] = "$partno is incorrect";
}
}
if (count($errors) > 0) {
die("Errors: " . implode($errors));
}
... proceed to "success" code ...
Basically, for every test that fails, you record a message. Once the loop exits, if there's any error messages, you display them and abort processing. If there's no errors, you proceed onwards with the rest of the code.

Answer 2

Why not use a try catch block?
try {
if (isset($_POST)) {
if (!$email) {
throw new Exception('email is not valid', 100);
}
// everything is good process the request
}
throw new Exception('Error!', 200);
} catch (Exception $e) {
if ($e->getCode == 200) {
// redirect
} else {
// do something else
}
}

Answer 3

throw an Exception in your If statement, then put your data in try/catch block, so it will catch an exception if error occured

Answer 4

Consider the following approach: both the form and the php code to do something with the form data are on the same page. If the form was posted, you'll first check if the form was ok, after that you'll do something with the submitted data. If the form was not valid, display an error message.
The advantage is: no die() in the middle of your code, no strange redirects, everything in one script.
// simplified code in example.php
<?php
// in this variable we'll save success/error messages to print it
$msg = "";
// run this php code if the form was submitted
if(isset($_POST['submit'])) {
// is the form valid?
if (strlen($_POST['username']) == 0) {
$msg = "Please enter a username";
}
else {
// all validation tests are passed, give the user a nice feedback
// do something with the data, e.g. save it to the db
$msg = "Your data was saved. Have a great day";
}
}
?>
<div class="msg"><?php print $msg; ?></div>
<form method="post">
<input type="text" name="username">
<input type="submit" name="submit" value="Submit">
</form>